1. Field of the Invention
The present invention relates to an arrangement structure of an electrically-driven actuator used in a variable valve operating mechanism in which an operation of at least one of an intake valve and an exhaust valve of an engine can be adjusted.
2. Description of the Related Art
Many valve operating mechanisms of engines mounted on automobiles are constituted such that a valve opening/closing timing or a valve opening period of an intake valve or an exhaust valve can be adjusted from the viewpoint of exhaust gas measures or the like.
As an example of a structure of the variable valve operating mechanism, a structure is proposed in which a displacement of a cam provided on a camshaft is transferred to an oscillating cam of a reciprocating type by using a center rocker arm, and an intake valve or an exhaust valve is driven by a rocker arm driven by the oscillating cam. The oscillating cam has a shape in which a base circle section and a lift section are continuous with each other.
The posture of the center rocker arm is adjusted by the actuator. When the posture of the center rocker arm is changed, a position at which the center rocker arm is in contact with the cam is changed and a position at which the center rocker arm is in contact with the oscillating cam is also changed. As a result, the operation of the intake valve or the exhaust valve is changed. A variable valve operating mechanism of this type is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2005-299536.
An electrically-driven actuator represented by an electric motor is employed as the actuator. An actuator of this type is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-332549.
It is desirable that an electrically-driven actuator of a variable valve operating mechanism be accommodated in a cylinder head cover in order to make the engine compact.
However, the above-mentioned valve operating mechanism is accommodated in the cylinder head cover, and hence it is difficult to accommodate the electrically-driven actuator in the cylinder head cover. Thus, a part or an entire part of the electrically-driven actuator is exposed to the outside of the cylinder head cover in some cases.
On the other hand, when an automobile is moving, heat generated from the engine, heat generated from the transmission, and heat generated from the radiator core is cooled by a wind caused by an air blast of a fan or movement of the car.
When the engine is stopped, the wind caused by the air blast of the fan or the movement of the car disappears. As a result, after the engine is stopped, the heat generated from the engine, the heat generated from the transmission, and the heat generated from the radiator core stagnates in the engine room. In the engine room, a flow path of the heat is generated. Specifically, the flow path is a flow path of heat in which the stagnating heat, i.e., the heated air first moves to an upper part in the engine room, and then moves toward the rear part of the car body along the slant of the hood member. In this case, if the part of the electrically-driven actuator exposed to the outside of the engine is arranged in the flow path of heat, the motor is heated by the heat. When the motor is heated, the coil resistance inside the motor is increased or the magnet is demagnetized. As a result, the performance of the motor is lowered and the control response is deteriorated.
If the control response is deteriorated, the effect of the variable valve operating mechanism is deteriorated, and hence it is undesirable that the motor is heated.
Thus, the capacity of the motor is set large in some cases. However, if the motor capacity is increased, the mountability of the actuator is deteriorated by an increase in the motor size. Further, the cost of the motor is increased due to the need to enhance the performance of the magnet used in the motor. Further, by heating the motor, the lubricity of the grease inside the motor is deteriorated, and hence the durability of the motor is also deteriorated in some cases.